MT TWI

The Welding Institute 1.

INTRODUCTION 1.1

History of Magnetic Particle Inspection The origins of magnetic particle inspection can be traced to the 1920s although the method did not become fully accepted until the expansion years of World War II. In the early days battery packs and direct current were the norm and it was some years before alternating current proved acceptable. Magnetic particle inspection superseded the oil and chalk method in the 1930s as it proved far more sensitive to surface breaking flaws. Today it is still preferred to the penetrant method on ferromagnetic material.

1.2

Magnetism The phenomenon called magnetism is said to have been discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. The use of magnets in navigation goes back to Viking times or maybe earlier, where it was found that rods of magnetised material, when freely suspended, would always point in a north-south direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, to be followed by Michael Faraday (1791-1867) whose experiments revealed that magnetic and electrical energy could be interchanged.

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The Welding Institute 2.

THE PHYSICS OF MAGNETISM 2.1

DIAMAGNETISM AND PARAMAGNETISM All materials are affected by magnetic fields, to a greater or lesser degree. The change or orbital motion of the electrons in the atoms of the substance concerned relates to the degree of magnetisation. Those materials which are: a)

Repelled by a magnetic force are called DIAMAGNETIC. They have a small negative susceptibility to magnetism.

b)

Lightly attracted by a magnetic force are called PARAMAGNETIC. They have a small positive susceptibility to magnetism.

c c) termed

Materials strongly attracted by a magnetic field are FERROMAGNETIC

2.2

FERROMAGNETISM AND DOMAIN THEORY Ferromagnetic materials are strongly attracted by magnetic fields. These are the materials which can be magnetised and thus tested by magnetic particle inspection. Ferromagnetism can be explained using the idea of the magnetic domain. Domains can be considered to be minute internal magnets, each perhaps comprising 1015 to 1020 atoms. In ferromagnetic atoms, the configuration dictates that more electrons spin one way than the other. The resultant magnetic moment of a group of atoms means that an internal polarity is created. Simply, a very small internal magnet having a north and south pole.

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The Welding Institute Unmagnetised state. Domains randomly orientated.

Field Magnetised state. Domains orientated in external magnetic field.

Field Field Saturated state. Domains orientated in strong external field.

Field

Residual state. Domains remaining orientated in absence of external field.

Field Demagnetised state. Domains randomly orientated in opposing field Field Figure 1: Domain Theory

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The Welding Institute 2.3 PERMANENT MAGNETISM When the external magnetising force is removed from a ferromagnetic material the domains will remain in a partial alignment dependent on a number of factors, such as: Alloying elements Carbon content Heat treatment Temperature Strong permanent magnets used in magnetic particle inspection are commonly made of iron alloyed with aluminium, nickel and cobalt. Hence such trade names as: Alnico or Alcomax. If a bar magnet is placed under a flat sheet of paper and iron filings are sprinkled on to the paper, a visual field is created. This is called a magnetographThe filings are orientated by the magnetic field created by the lines of force running between the poles of the bar magnet.

S

N

Figure 2: Permanent Magnet Shown below are a number of rules relating to lines of force: 1.

By convention flow from north to south outside the material and south to north inside

2.

Do not cross

3.

Repel each other laterally

4.

Are in a constant state of tension

5.

Are more numerous where the field intensity is greatest

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The Welding Institute 2.4

ELECTROMAGNETISM When an electric current flows through a conductor, a magnetic field is set up around the conductor in a direction at 90° to the electric current. This is explained by the 'right hand rule', see Fig.3. If the thumb of the right hand is extended in the direction in which the current is flowing, then the direction of the magnetic field is represented by the fingers.

Figure 3: Right Hand Rule When the conductor is ferromagnetic, strong magnetic flux lines are created also in the direction of the fingers. This is called circular magnetism. Circular magnetism is not polar and cannot be detected externally on a round symmetrical specimen. Now, if the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction.

Figure 4: Coiled conductor

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The Welding Institute The field within the loop has direction and one side will be a north pole and the other a south pole. By increasing the number of loops, a coil, or solenoid, is created and the strength of the field passing through the coil is proportional to the current passing through the conductor in amperes multiplied by the number of turns in the solenoid, see Fig.5a. When a ferromagnetic specimen is placed in an energised coil, the magnetic field is concentrated in the specimen. One end of the specimen is a north pole and the other south pole. This is called longitudinal magnetism, see Fig.5b. Longitudinal magnetism has polarity and is therefore readily detectable. Only one type of field can exist in a material at one time; the stronger will wipe out the weaker. Normally in magnetic particle inspection, circular tests are done before longitudinal.

Figure 5a: Coil Magnetisation

Figure 5b: Coil Magnetisation

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The Welding Institute 2.5

MAGNETIC HYSTERESIS When a ferromagnetic material is influenced by an alternating magnetising force (H), the variation of magnetic flux density (B) in it is related to a phenomena known as magnetic hysteresis. The word hysteresis is derived from the Greek word for delayed and is used to describe one quantity lagging behind another. The variation of B to H follows a hysteresis loop and is characteristic to a particular ferromagnetic material. Figure 6 is a typical hysteresis loop whose co-ordinates represent magnetising force (H) on the horizontal axis and flux density (B) on the vertical axis.

B

a

b

c

f o

H

e d

Figure 6: Hysteresis When an unmagnetised ferromagnetic material is exposed to a gradually increasing magnetising force the corresponding flux density can be plotted along the dotted line o - a. The level of flux density is increased until point “a” is reached and a further increase of magnetising force produces no increase in flux density. The specimen is saturated with flux and indeed, point 'a' is called the saturation point.

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The Welding Institute The dotted line o-a is often referred to as the virgin curve. Point “a” towards point “b” is where the hysteresis loop begins. As the magnetising force is reduced the flux density does not fall back to zero but follows the line “a-b”. So at “b” there is a zero magnetising force but a flux density “o-b” remains. The flux is lagging behind the force and this is what gives ferromagnetics their permanent magnetism. To reduce the flux density to zero, or demagnetise the specimen, a negative magnetising force “c” has to be applied and maintained. So as the force increases to produce this the relationship of B to H follows the line “b” to “c”. The force “o” to “c” required to demagnetise the specimen, is called the coercive force. Increasing the negative magnetising force still further produces a B to H relationship along the curve “c” to “d”. Point 'd' is exactly opposite point “a” and represents negative saturation. As the negative force is reduced, point 'e' is reached, exactly opposite point “b” and reversal to a positive magnetising force achieves a zero flux density at point “f”, exactly opposite “c”. The loop is completed by increasing the magnetising force, giving a B to H ratio along curve “f” to “a”. Note that once the virgin curve is produced the hysteresis loop does not pass through “o” again. The specimen will not be demagnetised until special steps are taken to achieve that state. As stated earlier, a hysteresis loop is characteristic to a particular ferromagnetic material.

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The Welding Institute Figure 7: 2 Loops Relating to Iron and Steel. Alloying Element Silicon

Hysteresis

Permeability

Coercive Force

Remanence

Manganese with pearlitic steels Manganese with austenitic steels Chromium Nickel with pearlitic steels Nickel with austenitic steels Aluminium Tungsten Vanadium Cobalt Molybdenum Copper Sulphur Phosphorus

Increase ( the more arrows the more intense the effect ) Decrease Unknown Table 1: Effect of Magnetising Elements

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Loss of Power

The Welding Institute 2.6 DEFINITION OF TERMS A knowledge of some of the physical terms related to magnetisation is essential. However, the following definitions are not meant to be exhaustive but only those which are considered relevant to understanding the practice of magnetic particle inspection. BS3683:Part 2:1985 provides a comprehensive glossary. Flux density The number of magnetic flux lines per unit area Symbol = B SI unit = tesla = T (It has replaced the gauss and 1 tesla = 104 gauss) Magnetising force The total force tending to set up a magnetic flux in a magnetic circuit. Symbol = H SI unit = ampere per metre = Am-1 (It has replaced the oersted and 1Am-1 = 0.01256 oersted or 1 Oe = 79.58Am-1). Permeability The ease with which a magnetic field or flux can be set up in a magnetic circuit. Symbol = µ (mu) µ =

B H

µ is the absolute magnetic permeability in Henry/metre B is the magnetic flux density in tesla (T) H is the magnetic field strength in amperes per metre (A/M) For air and non-magnetic materials, µ is constant and denoted by µ° µ° = 4π x 10-7 Henries/metre For ferromagnetic materials it varies considerably according to the value of H. For convenience we use relative permeability µr: µr

µ

= µ

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The Welding Institute

Relative permeability is therefore a dimensionless ratio which relates the permeability of the material to that of air. Saturation The stage at which any increase in the magnetising force H applied to a specimen, produces no significant gain in flux density B. Effectively it is at point 'a' on Fig.6. Saturation on a test specimen, can be recognised by a high ink background caused by clumping, furring or blushing of the particles. Coercive force The reverse magnetising force required to remove residual magnetism from a material. On Fig. 6 it is represented by 'o-c'. Remanence The magnetic flux density remaining in a material after the magnetising force has been removed. On Fig.6 it can be any value of B , between b and e, when H = O. Residual magnetic field The magnetic field remaining in a material after the magnetising force has been reduced to zero. Reluctance A measure of the degree of difficulty with which a component can be magnetised that is analogous to resistance in an electrical circuit. It is the reciprocal of permeability. Retentivity The magnetic flux density remaining in a material after the magnetising force has been removed (BS3683 Pt.2), synonymous to remanence. However, McGraw-Hill, "Dictionary of Scientific and Technical Terms", defines retentivity as, the residual flux density corresponding to the saturation induction of a magnetic material. This corresponds to point 'b' in Fig.6, the maximum remanence.

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The Welding Institute 2.7

FLUX LEAKAGE A flux leakage is a break or a discontinuity in a magnetic circuit. Any abrupt change of permeability within a magnetic specimen will change the number of flux lines which can flow and thus there will be a diversion of the field. Magnetic particle inspection relies on flux leakages being seen on the surface of a ferromagnetic specimen under test. All defects produce flux leakages but not all flux leakages are created by defects. Magnetic particle inspection relies on: a)

Magnetising the specimen to an adequate flux density.

b)

Applying fine ferromagnetic particles over the surface of the specimen.

c)

Being able to see clusters of the magnetic particles which gather at flux leakages.

The magnetic field must run in a direction so that it can be interrupted by the defect, thus producing a flux leakage field. Also the degree of distortion at the leakage must allow the magnetic particles to provide an adequate degree of contrast between the leakage and the adjacent material surface, so that it is readily visible. Flux lines will take the path of least reluctance, hence the highest permeability. Figure 8a shows flux lines flowing in a ferromagnetic bar but having to divert around an air gap, creating a flux leakage. However, if ferromagnetic particles are sprinkled on the bar they will start to form a magnetic bridge across the flux leakage and a highly preferred path. If the flux leakage is strong, such as a surface breaking crack in the optimum direction, then the visual indication will be plain. See Fig.8b. a.

b.

Figure 8: Flux Leakage

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The Welding Institute Whether a flux leakage is made into a visual indication depends on a number of factors, such as: a) b) c) d) e) f)

Size of defect Shape of defect Volume of defect Orientation of defect Depth below surface Permeability of material

2.7.1 Indications Indications are any particle indications which are seen on the specimen under test. Just as not all flux leakages are defects, not all indications are flux leakages. Indications can be further subdivided into: 

Relevant



Spurious



Non-Relevant

2.7.1.1 Relevant indications Relevant indications are discontinuities or flaws, which in turn are undesigned imperfections. When it is considered that a relevant indication will affect the fitness for purpose of a test specimen, then it is classified as a defect, but not all defects are cracks. Product and process knowledge (a knowledge of product technology and the processes that a test specimen has been through) is necessary to define and interpret defects more closely. It is perhaps safer, without that knowledge, to categorise indications by their: a) b) c)

Size Shape Orientation

2.7.1.2 Spurious indications Indications which are not held on the surface by a flux leakage are called spurious. Lint, scale, dirt, hairs, drainage lines, etc. are examples. TWI Training & Examination Services – NDT 30M 13

The Welding Institute However, there is one spurious indication called magnetic writing which is a little different. If two pieces of steel touch when one of them is in a magnetised condition local poles are created at the areas of contact. If magnetic particles are then sprinkled on the surface the local poles become visible as fuzzy lines. 2.7.1.3 Non-relevant indications Non-relevant indications are true magnetic particle patterns actually formed and held in place by leakage fields. However, they are caused by design features and the structure of the specimen and only exceptionally will they affect the fitness for purpose of the specimen. Below is a non-exhaustive list: a) b) c) d) e) f) g) h) i) j) k)

Scores and scratches Key ways Internal splines and drillings Abrupt changes of section Fine threads Force fits Dissimilar magnetic material (HAZ or heat treated material) Forging flow lines Grain boundaries Brazed joints Cold working

2.7.2 LONGITUDINAL FIELD It has already been said that the magnetic flux lines must run in a direction so that they can be interrupted at a defect causing a flux leakage. So, in order to detect defects, the flux lines should ideally be at 90° to the direction of potential defects. In Fig.9 the magnetic lines of force are longitudinal in a bar and thus the bar has magnetic poles. Transverse flaws will easily show; but longitudinal defects such as seams, which are very straight, will not show. However, it is accepted that flaws up to 45° to the flux lines will also be shown. In fact, longitudinal flaws having a transverse component, such as jagged cracks, will almost certainly show.

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The Welding Institute Magnetic Lines of Force

Magnetising Current

Longitudinal Defect will not show

45o Defect may show

Transverse Defect will show

Figure 9: Longitudinal Field 2.7.3 CIRCULAR FIELD The longitudinal magnetising field in the bar is now replaced by a longitudinal current, which creates a magnetic field at 90° to itself. In fact, the current has produced a circular non-polar field around the bar. Under normal circumstances the circular field is not detected, due to it having no external poles, but a longitudinal surface flaw at 90° creates a flux leakage, creating miniature poles and is thus detectable with magnetic particles. Fig.10 shows the effect of flaw orientation in circularly magnetised bar. Magnetic Lines of Force

Magnetising Current

Longitudinal Defect will show

Figure 10: Circular Magnetism TWI Training & Examination Services – NDT 30M 15

45o Defect may show

Transverse Defect will not show

The Welding Institute 3.

EQUIPMENT The equipment used for magnetic particle inspection can be divided up according to size and purpose. The magnetising force may be supplied by anything from a small permanent magnet to a highly sophisticated fixed installation, utilising high values of rectified current and finely calibrated meters. When electricity is introduced into a specimen in order to magnetise, it is usually transformed into a low voltage, high amperage supply. Therefore there is no danger from electrocution, however, specimens do get hot due to electrical resistance if the supply is applied for more than a couple of seconds. 3.1

Permanent magnets Permanent magnets produce a longitudinal magnetic field between the poles. Modern variants of the horseshoe magnet have adjustable arms, and may have variable geometry removable pole ends. Optimum defect detectability is at 90° to the poles. Modern opinion tends not to favour permanent magnets.

N

S

Figure 11: Permanent Magnets Advantages      

No power supply needed Cling to vertical surfaces No electrical contact problems Inexpensive No damage to test piece Lightweight

Disadvantages  Direct field only  Deteriorate with wear  Have to be pulled from test surface  No control over field strength  Magnetic particles attracted to poles  

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The Welding Institute 3.2

Electromagnets Electromagnets are made from soft iron laminates to reduce eddy current losses, if powered by alternating current (AC). The yoke laminates are encased in a multi-turn coil, usually powered by mains electricity. The legs of modern equipments are normally articulated to allow area contact on uneven surfaces.

Figure 12: Electromagnet Electromagnets produce a longitudinal field. Defect orientation is the same as when using a permanent magnet. Rectified AC current or DC current from a battery may be used. DC is not favoured as a magnetising method, due to the possibility of not achieving an adequate flux density within the specimen. Advantages        

AC, rectified or DC operation Controllable magnetic field strength Run direct from mains electricity supply Can be switched on and off allowing Easy removal No harm to test piece Lightweight Can be used to demagnetise on AC

Disadvantages  Needs power supply  Longitudinal field only  Carry mains voltage  Poles attract magnetic particles  Legs must have area contact

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The Welding Institute 3.3

Prods Prods induce a circular magnetic field by sending a high amperage (1000A typically) current through the test piece. The high amperage can cause arcing between the electrodes and test surface. Contact points must be carefully cleaned, and electrode materials chosen to prevent contamination of the test piece.

Figure 13: Prods Advantages    

Variable field strength AC or DC fields Useful in confined spaces Low voltages



No poles to attract particles Control of amperage



Disadvantages    

Danger of arcing Danger of overheating Heavy transformer required Possible to switch on without creating field  Possible contamination of the test piece by the electrode  

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The Welding Institute 3.4

Flexible coil In this technique the current-carrying cable is wound tightly around the component. It is a longitudinal magnetisation method and will find defects lying parallel to the cable. If possible the turns should be spaced so that inspection is possible between them. Split coils with quick release fasteners are commercially available to allow coils to be fixed and removed more quickly. Advantages   

Simple to operate No danger of burning AC and rectified current



Magnetising force is the product of amps needed multiplied by turns Current is adjustable

 3.5

Disadvantages  Difficult to keep turns apart  Limited inspection cover  High current capacity sometimes

Flexible cable and close loop Working from the basic principle that a current must create a magnetic field around the conductor, the flexible cable is a useful means of testing welded constructions, large casting and forgings. The cable may be laid on to a job, perhaps parallel to a weld and defects will be found in the direction of current flow, adjacent to the cable. When used as a single or a multi-turn threading cable, the conductor is passed through openings of interest and defects will be found radially around the hole or longitudinally in the bore. When used on a pipe, defects will be found parallel to the cable internally and externally, as well as radially on the ends. The parallel closed loop is a novel variation which has found some favour in underwater inspection and the gas industry. a)

The cables or conductors are kept apart by insulators

b)

The direction of current in each cable must be complementary, not opposing

c)

Defects will be found within the grid, parallel to the conductors.

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The Welding Institute Advantages    3.6

Simple application Variable field strength Can cover large areas

Disadvantages  May require long cables  High current draw sometimes  Difficult to keep cable in position

Clamps and leeches Where prods are not suitable because heat damage may be caused, or the item is too large and awkward, it is often still possible to pass a current into a specimen. Special 'crocodile' clips with copper woven braiding on them are one alternative. Another possibility is to use permanent magnets as leeches to clamp on the job so that the operator's hands are free to apply the ink or powder. The current is passed through the leeches and does not affect the permanent magnetism.

3.7

Mobile Equipment As the name implies, mobile equipment is too bulky and heavy to carry and yet needs to be moved to the work. Some mobile units are capable of supplying output currents up to 20,000A, although 5000A is a more normal figure. The current required to test a job may be quite low but losses due to cable length or bulk of specimen may mean that a portable set cannot produce enough. Paragraphs 3.3 to 3.6 are relevant to mobile units as well as portables. In addition to the normal features on a portable unit, the mobile is likely to have better current control and a step control to allow demagnetising.

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The Welding Institute 3.8

Bench Units Bench units are fixed installations used to test large numbers of manufactured specimens. They range in size and output from those able to test small components at no more than a few amperes to large cranks and gun barrels capable of 10-20,000A. Essentially, the electrical components of the mobile are incorporated in the bench unit together with handling and operating features that make testing more rapid, convenient and efficient. Among the features normally found on a bench unit are: a) b) c) d) e) f) g) h) i) j) k) l)

Adjustable head and tail stocks on a fixed bed. Manually, electrically or air operated. Agitated ink trough or reservoir. Recirculating ink supply from reservoir to spray gun. AC and rectified current facilities. Large area copper gauze covered electrodes on head and tail stock to allow current flow, for circular magnetisation. Magnetic flow solenoids on head and tail stocks and/or rigid coil on the bed, for longitudinal magnetisation. Calibrated meters. Controls to vary magnetising force values. Foot switches and hand switches to operate controls. UV-A (black light) lamp (optional). White light lamp (optional). Timers, to adjust operating duty cycle (optional).

3.8.1 Current Flow The component is fixed firmly between contact heads which have a soft conductive surface, such as copper braiding. A low voltage, high amperage current is passed through the component creating a circular magnetic field around it. The method favours detection of defects lying in line with the contact heads and not more than 45° from the ideal. Field

Current

Figure 14: Current Flow TWI Training & Examination Services – NDT 30M 21

The Welding Institute 3.8.2

Threading Bar Magnetisation by the threading bar technique is induced by passing current through an insulated non-magnetic conductor (aluminium, copper or brass are usual) which is placed in a bore or aperture in the component. Hollow components such as tubes and rings, are normally tested by the threading bar technique. In practice a number of small parts, such as rings, can be tested at the same time, providing they are not allowed to touch each other.

Current Figure 15: Threading Bar The threading bar technique induces circular magnetisation and defects in the same direction as the current will be found, externally, internally, and on end faces. Defects deviating up to 45° from the ideal will also be found. 3.8.3 Magnetic Flow Energised solenoids in the bench heads create a longitudinal magnetic field in a component, which is clamped between the heads, completing the magnetic circle. Defects, where the major axes lie transverse to a line joining the heads, are found best and the method is most applicable for short simple shapes. The solenoids on bench equipment are energised by full wave rectified current.

Magnetism

Figure 16: Magnetic Flow TWI Training & Examination Services – NDT 30M 22

The Welding Institute Where there are large differences between the size of the bench heads and the ends of the component, shaped extenders may have to be used to ensure that the flux is smoothed into the ends of the component. If this is not done, clumping of magnetic particles on the component will prevent defect detection. 3.8.4 RIGID COIL The component is placed in a current-carrying rigid coil with its longitudinal axis at 90° to the direction of the windings on the coil. Four to eight turn coils are usual and the specimen is placed in the bottom of the coil wherever possible. Longitudinal magnetism is induced into the component, so the method favours basically transverse defects. Long and slender components are best tested in coils, although a long component may have to be re-tested along its length. 300mm is about the longest that one can expect to inspect at any one time. Pole extenders should be used on short components, having a length/diameter ratio less than 5:1.

Current

Figure 17: Rigid Coil

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The Welding Institute 3.8.5 Induced Current The induced current technique is not normally a feature of standard bench equipment but is applicable to particular components, such as high quality finish bearing races, where arcing would ruin the part. The technique induces a circumferential current flow in a ring specimen by making it the secondary winding of a transformer. Therefore only alternating current may be used and only surface defects are revealed. It is a novel but extremely useful technique, as it eliminates the possibility of overheating the component under test. There are many variations, as often the technique has to be tailored to suit a specific component's inspection need.

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The Welding Institute 4.

CURRENT WAVEFORMS It is the value of peak current which creates the maximum magnetising force and therefore the most drive to the magnetic particles to migrate to a flux leakage. However, few ammeters are calibrated in peak values. In fact they read some other quantity such as root mean square (rms), mean or average. The apparently simple ampere can be read by a meter in many different ways and it is necessary to be aware of this. It is intended to look at the more common current waveforms and the more usual ways of reading their outputs. In view of the many ammeter variations, the safest thing for operators to do is to check with the equipment manufacturer as to what type of ammeter is fitted. Then print the peak to actual readout ratio on the meter scale. 4.1

DIRECT CURRENT An electrical current flowing in one direction only and effectively free from pulsation. Therefore, after a small build-up period the current is at a constant peak value and this is what the meter reads.

+ Figure 18: Direct Current Direct current is either supplied from a battery pack or a DC generator. In the early days of MPI, DC was almost universally used. This is not so today. Advantages  

Sub-surface defects Availability from batteries

Disadvantages  No agitation  Less sensitive to surface defects

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The Welding Institute 4.2

Alternating Current Alternating current is a form of electricity which, after reaching a maximum value in one direction, decreases, reverses direction and reaches a maximum in the opposite direction before returning to zero. It is cyclic and the cycle is repeated continuously.

+ Figure 19: Alternating Current It is of course, the peak current which creates the maximum magnetising force, but in reality the meter reads the root mean square (rms) value as the current is reversing between equal but opposite peak values it is therefore impossible to measure the mean value. By plotting the squares of the current values we can find an average, since negative as well as positive values become positive. To measure the square of the current we use a moving iron ammeter. This type of ammeter consists of two iron rods which are forced apart as they are magnetised. Their level of magnetisation is proportional to the current and therefore the force between them is roughly proportional to the square of the current. The meter is calibrated to read the root of the mean of the square values and is therefore nonlinear. Advantages    

Availability Sensitivity to surface defects Agitation of particles Demagnetisation

Disadvantages  Will not detect sub-surface defects

The phenomenon that causes the magnetisation produced by alternating current to be contained near the surface of a ferromagnetic component is called skin effect. Therefore if the TWI Training & Examination Services – NDT 30M 26

The Welding Institute magnetic field produced by AC only exists at or just under the surface of the component and only surface defects will be revealed by AC. If subsurface defects are of interest, rectified or DC current must be used because they produce an even flux density through the cross section of the component. 4.3

Half wave rectified (HWR) This is a pulsed unidirectional current produced by clipping a half cycle from single phase alternating current. As a result there are intervals when no current is flowing. It is the least expensive form of rectification, used on cars and motorcycles. In the UK it is common in portable, mobile and bench units.

+ Figure 20: Half Wave Rectified Current Advantages 

Penetration like DC



Agitation

Disadvantages  Lower sensitivity to surface defects than AC

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The Welding Institute 4.4

Full wave rectified current (single phase) This is a form of current where the negative half wave of an alternating current is converted into a positive wave, so that both halves of the swing are able to deliver unidirectional current. Full wave rectified equipment is unlikely to be even nominally portable, due to the weight of electrical equipment within them. Bench units using this waveform are most likely to be found where codes from the USA prevail.

+ Figure 21: Direct Current Advantages 

Penetration like DC



Agitation

Disadvantages  Lower sensitivity to surface defects than AC

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The Welding Institute 5. MAGNETISING VALUES United Kingdom magnetic particle inspection practice is based on research which recommends that a minimum flux density of 0.72T must be achieved. Since most engineering steels have a permeability greater than 240 then an applied field greater than 2400A/m is required. The magnetising values quoted in this test are therefore derived with the above parameters in mind. It has been explained that different current wave terms are used in MPI, but not why. Alternating current is simple to transform, when taken from the electrical mains, and employ. Because the polarity is changing fifty times a second the magnetic particles are constantly reversing their direction and this causes them to migrate or 'walk' to flux leakages. This is excellent because it gives bright clear indications. However, because of the skin effect phenomenon the magnetism is concentrated near to the surface of a component. Therefore only surface defects can be found using alternating current. It is considered that subsurface defects are critical and it is believed that they are likely to be orientated in a way that makes detection possible, then DC or rectified current must be used. Good HWR circuits will give full depth magnetic penetration with a pulse effect to help the particles migrate. Complicated machined specimens with fine threads or key ways might be difficult to interpret due to flux leakage across changes of section. It is often possible to use the residual magnetism to produce fine line indications and reduce the incidence of non-relevant indications. This is called the residual technique and when employed, rectified current or DC must be used. Thus, before selecting a magnetising value and wave form for a job, the type, orientation and depth of likely defects must be deduced. 5.1

Permanent magnets and dc electromagnets Permanent magnets and dc electromagnets should be able to lift at least 18kg of ferritic steel with the poles between 75 and 150mm apart. For pole spacing less than or equal to 75mm the magnet shall be capable of lifting not less than 0.24Kg/mm. Each pole should pull off at more than 9kg.

5.2

AC electromagnets An AC powered electromagnet must be able to lift 4.5kg with the poles 300mm or less apart. Each pole must pull off at more than 2.5kg. The test area is a circle inscribed by the poles.

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The Welding Institute 5.3

Prods If a flat area is to be tested then a pattern similar to that in Fig.22 is used. The peak current values must be not less than 7.5A/mm of prod spacing, equivalent to 5.3A/mm AC rms. When a simple check of a narrow region, such as a weld is required then the current value can be reduced to 3.75A/mm peak (2.7A/mm AC rms). Practically, on a weld, to cover both USA and UK requirements, a current of 100A/in. (4 A/mm) is adequate, with a maximum prod spacing of 200mm (8in.).

CF1

CF1

CF3

CF2

CF2

CF3

Figure 22: Prod Test Pattern 5.4

Flexible coil British Standard 6072 gives two formulae to be used when testing with a flexible coil. These are shown as follows: a) Using direct or rectified current, the peak value of the current flowing in a cable shall have a minimum value of:  Y 2   1 = 7.5 T +     4T   

where I = peak current T = wall thickness (in mm) or radius of component if round Y = the spacing (in mm) between adjacent windings in the coil. b) Using alternating current, the peak value of the current flowing in the cable shall have a minimum value of: TWI Training & Examination Services – NDT 30M 30

The Welding Institute

  Y 2  I = 7.5 10 +     40   

where I = peak current Y = the spacing (in mm) between adjacent windings in the coil The UK system requires the cable windings to be spaced. But in the USA it is accepted that spacing the windings is extremely difficult and thus the formula in ASME V shown below applies to flexible close turn coils: N x I =

K L +2 D

where: I = coil current N = number of turns in the coil or cable wrap L = part length D = part diameter K = 35000 Note: The maximum L/D ratio for calculations is limited to 15:1. The effective field extends on either side of the coil to a distance approximately equal to its radius.

5.5

Flexible cable To achieve most efficient magnetisation the cable should be mounted at a distance from the test surface (d), see Fig. 23. The width of the effective inspection area on each side of the cable

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The Welding Institute centreline is then also d and is related to the peak current value (I) in the cable by:

I = 30d r d2 d1

d1

Figure 23: Flexible Cable r = radius of influence d1 = distance from surface d2 = distance of effective cover This formula can be extended to give a simple close wrapped flexible coil formula, where N x I = 30d N being the number of turns.

Figure 24: Flexible Coil 5.6

Current flow The values for current flow applications are given in Tables 2 and 3. When components having varying cross section are tested, a single current value can be used if the diameters of the larger and small

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The Welding Institute sections are within a ratio of 1.5:1. The large diameter governs the value. If the cross section variation is greater than 1.5:1 then each section is tested in turn, starting with the smallest. Current Waveform

Peak or DC Source

AC Source B

Single Phase Full Wave Rectified A

Single Phase Half Wave Rectified B

Type of ammeter Current for Basically Round components per mm of diameter Current for Non-Round components per mm of perimeter

A 7.5

5.3

4.8

2.4

2.4

1.7

1.5

0.75

Table 2: General Engineering Current Values Current Waveform

Peak or DC Source

AC Source

Type of ammeter Current for Basically Round components per mm of diameter Current for Non-Round components per mm of perimeter

A

5.7

B

Single Phase Full Wave Rectified A

Single Phase Half Wave Rectified B

28

20

18

9

9

6.4

5.7

2.9

Table 3: Aerospace Current Values Threading bar When the threading bar is placed centrally the current values given in Tables 2 and 3 may be used.

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The Welding Institute Alternatively, and when the threading bar is offset from the centre, the surface under test shall lie totally within a circle centred on the threading bar. The radius (in mm) (r) of this circle is given by: r=

r=

1 general engineering 15

1 56

aerospace application

where I = peak current value and where I = AC rms current value this becomes r=

r=

1 40

1 11

general engineering applications

aerospace applications When large rings, etc. have to be tested a number of shots, equidistant around the circumference may be necessary, see Fig. 25. Test 1 Test 2

Test 5

Test 3 Test 4 Figure 26: Threading Bar Test Coverage 5.8

Magnetic flow There are no formulae applicable to the magnetic flow technique. It is recommended that the magnetic field strength should lie between

TWI Training & Examination Services – NDT 30M 34

The Welding Institute the value which will just saturate the material under test and not less than one third of the saturation value. Alternatively, flux indicators are used. 5.9

Rigid coil The formulae given by BS6072 is: Ip =

32000 22000 or Irms = L L xN xN D D

Where Ip is the current peak value Irms is the rms current value N is the number of coil turns L is the testpiece length D is the testpiece diameter This can be usefully transposed to read: 22000 N x Irms = L/D

To use the formula, the following conditions apply: 1.

The cross-sectional area of the testpiece must be less than 10% of the cross-sectional area of the coil aperture.

2.

The test piece should lie against the side or bottom of the coil.

3.

L/D ratio of the part must be greater than 5:1 if not, pole extenders can be clamped to the ends of the testpiece.

4.

If the L/D ratio exceeds 20 then the ampere turn value for a 20:1 ratio should be used. The test should be repeated at coil length intervals. The major axis of the test piece should be parallel with the axis of the coil.

5. 6.

When using rigid coils of helical form the pitch of the helix shall be less than 25% of the coil diameter.

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The Welding Institute 7.

BS6072 implies that only the section in the coil is tested and the test must be repeated at coil length intervals. In US instructions the test area extends 6" beyond the coil on each side.

Formula: N= I= D= L= X=

number of turns amperage diameter of specimen length of specimen 32 000 for peak 22 000 for AC rms and FWR 11 000 for HWR

4

8000

5500

2750

5

6400

4400

2200

6

5333

3667

1833

7

4571

3143

1692

8

4000

2750

1571

9

3556

2444

1467

10

3200

2200

1100

11

2909

2000

1000

12

2667

1833

917

13

2462

1692

846

14

2286

1571

15

2133

1467

786 733

Table 5. Fixed coil current for known L/D ratios

5.10

Induced current A clamp meter is required to find out the value of current induced into a component. If one is available then the current values used for current flow apply. However, if the correct type of ammeter is not to hand, a flux indicator is the alternative.

5.11

Flux Indicators

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The Welding Institute Field strength meters based on the Hall effect are the best way of ascertaining adequate field strength at the surface of a test component. However, they are expensive and the probes used tend to be fragile. Portable flux indicators are an acceptable alternative. They: a) b) χ

are simple to use provide a clear visual indication of the direction of the surface field c) are a rough guide to the magnitude of the surface field.

This is only true if the flux indicator abuts intimately with the test specimen. Flux indicators consist of a magnetic material which is interrupted by non-magnetic spacers. When the flux indicator is placed on the surface of a magnetised specimen, flux is induced in it. The nonmagnetic spacers behave as artificial flaws. If the magnetic field at the surface of the specimen is sufficiently high, leakage flux above the artificial flaws can be detected by the application of a magnetic particle ink or powder. Flux indicators are made with high permeability magnetic materials with low coercivity and low remanence so that a flux can be easily induced into them, yet without permanently magnetising them. Opinion differs on their efficacy when used with permanent magnets and DC electromagnets. In every case when a permanent magnet or electromagnet is used, good area contact of the poles is imperative or the flux indicator is useless. Results may be misleading when indicators are used in a coil. Flux indicators may be divided into two main types: 

segment type



foil type

5.11.1 Segment type Four or eight identical segments of ferrous metal are joined with non-magnetic compound of even thickness into the shape of a flat disc. One surface of the disc is covered with non-magnetic foil to prevent magnetic particles getting to the surface and prevent them giving misleading indications. TWI Training & Examination Services – NDT 30M 37

The Welding Institute

The eight segment type, with a fixed foil is popular in the USA. A four section indicator with an adjustable foil, giving a varying air gap between them is called a Berthold penetrameter. 5.11.2 Foil type The most common foil type indicator is the Burmah Castrol strip. These indicators consist of a magnetic foil containing slots to simulate discontinuities, sandwiched between nonmagnetic foils. Non-magnetic foils are either brass or stainless steel depending on whether they are for general or aerospace use. The simulated discontinuities in a Burmah Castrol strip are arranged in three parallel lines. These foils are less than 0.2mm thick and flexible, which gives them a significant advantage over the segment type.

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The Welding Institute 6.

TEST METHODS The magnetic particle test method may be classified as 

Wet or Dry (based upon the detecting media used)



Continuous or Residual (according to when the detecting media is applied with relation to the magnetising force)



Visible or Fluorescent (according to the nature of the viewing conditions)

6.1

Dry Powders Powders comprise finely ground ferromagnetic particles, often iron, coated or heated to a temperature which will give a distinctive colour. British Standard 4069, magnetic flaw detection inks and powders, lays down that the particle size must be less than 200 microns. However, manufactures quote values well below this size. Ideally the particle shape should be elongated. However, to allow dry powders to flow from the dispenser, a mixture of rod shaped particles and globular ones is used. Typical colours for powders are: 

Black



Red



Grey



Yellow

Dry powders are dispersed on to the test component either through a puffer or a dry spray can. The chosen colour is the one which gives the best contrast against the specimen background. To enhance the contrast a white strippable contrast paint may be sprayed or painted on the specimen. If this is applied lightly, not more than 50 micron thick, there will be minimal effect on defect sensitivity. Powders are usually applicable to site work such as welds and castings, often as an initial check on a weld root pass, where wet materials would cause contamination. PD6513 states that dry powders can be used for testing hot components up to 300°c but fluorescent powders may lose their brightness if heated, so should be used at ambient temperature. We advise that manufacturers' recommendations should be followed. Invariably powders are treated as disposable and should not be re-used, due to the danger of contamination by dirt and moisture.

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The Welding Institute

6.2

Inks Magnetic inks are further sub-divided into: 

Contrast or fluorescent



Kerosene or water-based

The ink comprises finely ground oxides of iron, having high permeability and low retentivity, suspended in a liquid. British Standard 4069 requires the maximum particle size to be no more than 100 micron, however, contrast particles are typically less than 1 micron and fluorescent less than 10 micron. The reason for the difference is that the fluorescent particle is dye coated, whereas the contrast particle is a natural colour of red or black. Water-based inks are becoming more popular because of: 1. 2. 3.

price odour reduction health and safety implications

Inks are usually sprayed, flooded or ladled on to the specimen. Kerosene based materials are also sold in aerosols. Water-based inks when used on site are often sprayed from garden dispensers and when used in bench machines the tanks should be stainless steel. Although a wetting agent and corrosion inhibitor is added to the concentrate, the effects of contamination of the ink by corrosion products cannot be ignored. Water-based inks are sold as a concentrate and then mixed. Kerosene based inks are supplied in bulk but to maintain the solid content at the correct level a small amount of concentrate is added at intervals. It is not recommended that magnetic inks are made up with normal kerosene, especially fluorescent inks since: 

The fire risk is greater. BS4069 states a minimum flash point of 65°C



There will be a higher odour level



Almost certainly there will be a high background fluorescence under UV/A light.

Of paramount importance is the maintenance of the ink strength. The solid content must be constantly monitored as detailed in Control Checks, Section 9, and the ink must be constantly agitated to keep the solid content in suspension. The appropriate solid content levels TWI Training & Examination Services – NDT 30M 40

The Welding Institute

6.3

are 0.1 to 0.3% for fluorescent ink and 1.25 to 3.5% for visible (black) ink. Continuous technique The continuous technique implies that the detecting media is applied before the magnetising force, to a component and continued during the period of magnetisation. However, ink or powder application should be stopped before magnetisation is stopped. Indeed, on low retentivity components it is important to inspect at the same time as magnetisation and ink application. A classic case of reporting a spurious indication as a defect is where ink is allowed to run down the toe of a weld after a test. The solid content forms a visible line, exactly conforming to the shape of the toe and this line is often enhanced by the residual magnetism of the heat affected zone. What is worse is that the inspector notes the indication as spurious but fails to see small toe defects which are now masked by that spurious indication. To avoid overheating the component and the equipment, magnetisation times should be limited to 2-3 seconds. In fact some equipment has shot timers on them to avoid the duty cycle being exceeded. Table 5 lists the steps in a one shot continuous technique. It should be pointed out that if full cover of a component is envisaged, a number of shots will be required. 1.

Demagnetise if specified

2.

Clean

3.

Affix magnetising contacts

4.

Apply detecting media

5.

Apply magnetising force, 2-3 sec duration

6.

Stop detecting media

7.

Stop magnetising force

8.

Inspect - this should start at operation 5 and end at 8

9.

Demagnetise, if specified

10.

Clean

11.

Protect Table 5. Continuous technique

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The Welding Institute

If demagnetisation is called for, circular magnetising tests are done first followed by longitudinal. This is so because it is probable that a residual circular field is not detectable but that residual field will be removed by longitudinal test applied later. Therefore, the final residual field to be removed is a longitudinal one, which is detectable with a field indicator. All current waveforms are applicable to continuous techniques, depending on the defect morphology. 6.4

Residual technique The residual technique uses only DC or rectified forms of AC to magnetise a component because it is the residual flux density which is relied upon to attract magnetic particles to the flux leakages created by defects. Direct current and rectified AC produce a full cross section magnetisation, whereas AC will only create an effective flux density in the skin, hence skin effect. Thus, the residual field from AC is not considered adequate for the residual technique. Also the residual technique is only applicable on components which have high retentivity, that is high carbon equivalent steels. It usually follows that components suitable for the residual technique are high tensile machine parts, often when we are looking for flaws in corners or thread roots etc. If the continuous technique is used on these parts there will be a high build up of detecting media across such features and these non-relevant indications are likely to mask an actual defect beneath them. For best defect sensitivity the detecting media is applied after magnetisation and to allow time for the particles to migrate. Inspection takes place a short time after that. Table 6 lists the steps in a one shot residual technique. The magnetising values should be the higher ones recommended for aerospace, using the appropriate electrical current waveform. Again, circular magnetism shots should be done before longitudinal, as invariably demagnetisation will be necessary.

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The Welding Institute

1.

Demagnetise

2.

Clean

3.

Affix magnetising contacts

4.

Apply magnetising force, not AC, 2-3 sec

5.

Apply detecting media, spray or dip

6.

Wait, 30 sec-1 min

7.

Inspect

8.

Demagnetise

9.

Clean

10.

Protect Table 6. Residual technique

6.5

Fluorescence and the Electromagnetic Spectrum Fluorescence is the property of some materials to absorb electromagnetic energy of one wavelength and re-emit the energy at another . The ultraviolet and visible light section of the spectrum, which is of interest in MPI, lies between 100 and 800 nm, see Fig.27. (A nanometre (nm) is 1 millionth of a mm.) In MPI long wavelength ultraviolet (black light) light sources are used having a waveband between 315 and 400nm. This is UV-A radiation. Fluorescent inks absorb energy at approximately 365nm and re-emit at about 550nm. Industrial radiography Microwaves Ultra violet

10-10 10-8

10-6

Infra red

10-4 10-2 1cm Wavelength

Electric Waves

TV

102

104

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106

108

The Welding Institute Figure 27 : Ultraviolet spectrum 6.5.1 Types of UV-A Lamp By far the most common type of light source used to inspect components tested with fluorescent ink is the mercury vapour arc lamp. In fact, the mercury arc lamp is a street or workshop lamp which has a filter over it to reduce the visible light to a minimum but allow the UV-A to be transmitted. The filter is called a Woods in the UK and a Kopp in the USA. On the Philips type of lamp the filter is integral with the outer envelope but on the Magnaflux unit, using either a GE or Westinghouse lamp, it is separate. The mercury arc is drawn between electrodes enclosed in a quartz tube. The resistor limits the amount of current in the starting electrode. The quartz tube is mounted and enclosed in the outer glass envelope which serves to protect it and filter out any possible hazardous radiations. 400W mercury vapour arc flood-lamps can be used where very large components are tested or to give a background illumination in an inspection area. However, background light in a darkened area can be more economically provided by UV strip lights. 6.5.2 Safety Precautions and Operating Instructions Under normal working conditions, there are no known long term harmful effects arising from the use of UV-A (black light) sources, providing simple safety precautions and operating instructions are observed. The precautions and instructions in these notes are general. For full advice the manufacturers' data on a particular light should be followed. Safety precautions when using a UV-A mercury vapour arc lamp a)

Avoid looking directly at the light source

b)

The light must not be used without a correctly fitted filter

c)

Do not operate the light with a chipped or cracked filter

d)

Avoid contact with the lamp housing as it becomes hot

e)

Keep the light cables away from liquids, to avoid contamination or shorting

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The Welding Institute f)

Ensure that regular electrical earth continuity checks are carried out on the lamp unit 6.5.3 Operating instructions for a UV-A lamp a)

Allow 5 minutes warm up period after switch on, before inspecting with the lamp

b)

If the lamp is switched off and then immediately switched on again, allow a minimum of 10 minutes before recommencing inspection. The bulb will not relight until its temperature reduces.

c)

Avoid repeated switching on and off, as this will reduce bulb life significantly

d)

Angle the light with respect to the specimen being inspected, to avoid reflections which reduce inspection efficiency

e)

Clean the lamp filter regularly, with lint-free material moistened with a mild detergent/water solution

f)

Check the light output of the lamp regularly. This should be done in accordance with paragraph 6.1 of BS4489. The lamp must achieve a UV-A irradiance level of 0.8mW/cm2 at the testing surface.

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The Welding Institute 7.

DEMAGNETISATION British Standard 6072 recommends that demagnetisation should only be carried out if specifically requested. In certain industries the consequences of not demagnetising can be catastrophic. Demagnetisation should be carried out: 1.

before testing, if residual fields could affect test results

2.

between tests except for when a similar shot is to be applied but at a higher amperage. An exception can be made if a subsequent shot is to be applied at 90° to the original and the original field strength is to be exceeded

3.

after testing, when applicable

Post demagnetising must be done: 1.

on aircraft parts, where magnetic compasses and electronic equipment may be affected

2.

on rotating parts, where magnetic debris might adhere and cause excess wear

3.

where automatic arc or electron beam welding is to be carried out and arc wander may be caused by residual magnetic fields

4.

if residual magnetic fields may affect subsequent machining processes. Reamers and taps become magnetic as well and thus can break in use, if swarf is not cleared from flutes.

5.

when a high quality finish, such as electro-plating is to be applied. The particles attracted will prevent or reduce adhesion.

It is not usually necessary to demagnetise specimens which are to be heat treated. This is provided that the heat treatment is beyond the Curie point, about 700°c. At and above the Curie point, ferromagnetic materials become paramagnetic. Often it is not possible or practical to demagnetise a specimen completely, therefore a maximum residual field level must be agreed. An agreed deflection on a calibrated or uncalibrated magnetic field indicator is the most common. For critical situations a compass test is recommended. The component under test is positioned at an agreed distance from a suitable compass and rotated through 360°. The compass needle must deflect by less than 1°. TWI Training & Examination Services – NDT 30M 46

The Welding Institute 7.1

Principle of Demagnetisation Looking at a typical hysteresis loop for a ferromagnetic material Fig.6, after the initial magnetising force is applied and then removed, it is well nigh impossible to end the test with a zero flux density. Even if a negative coercive force is applied it will only keep the flux density at zero, as long as it continues to be applied. Figure 28 shows that the key to demagnetisation is that a reversing and reducing magnetising force must be applied, so that the hysteresis loop reduces until all the parameters achieve zero. There are a number of ways to achieve this.

Field Strength

Figure 28: Demagnetisation Process 7.2

Methods of Demagnetisation 7.2.1 Aperture coil, removal The component is passed through an aperture type coil, whose major axis is aligned in an east-west direction and which is carrying AC. The component is removed from the coil to a minimum distance of 1.5m before the current is switched off. Special demagnetisers of this type are usually multi-turn coil, working directly from a single phase AC supply. However, a hand held coil made from a portable unit cable may be adequate for site use. If the component cannot be passed through the demagnetising coil there is no reason why the coil should not be passed over the component to achieve the same result.

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The Welding Institute 7.2.2 Aperture coil, reducing AC Where it is not possible to remove either the component or the coil from the vicinity of each other, then the AC can be reduced to zero to achieve the same demagnetising effect. Modern units use a capacitor discharging to achieve an almost instantaneous result. 7.2.3 Aperture coil, reversing DC Sometimes if a component has been magnetised using DC or rectified AC, it is nearly impossible to reduce the residual flux density to a satisfactory level using AC. This is especially true if the component is a complex shape. Therefore, a reversing and reducing DC, or more usually full wave rectified and smoothed AC, is used. The component is usually left in the coil but with long components the operation is carried out several times along its length. Each reduction of current should be 50% of the preceding one, down to a reasonable minimum. 7.2.4 Electromagnet, reversing DC The same principles apply as with the reversing DC aperture coil method, but in this case the component is clamped between the poles of an electromagnet in a field strong enough to saturate it magnetically. The field is then reduced and reversed in 50% increments to near zero. 7.2.5 Electromagnet, AC yoke A most useful way to remove local residual fields on components in situ, on a structure than cannot easily be moved or removed, is by means of a portable AC powered electromagnet. The energised yoke is pulled over and off the component, to a distance of about 450mm and then switched off. If the level achieved is not adequate, the operation is repeated in the same way and direction until the residual field is removed.

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The Welding Institute 8.

CONTROL AND MAINTENANCE CHECKS In order to ensure that the equipment ancillaries and materials are up to standard it is necessary to carry out a number of control checks. It is also important to make sure that the system performs consistently each day. Common sense dictates that the equipment, etc is maintained properly. The checks covered in this section are meant to be guides to proper practice. In different organisations there will obviously be variations and therefore the code or standard specified for a particular job must be the overriding factor. 8.1

Ink settlement test This test is carried out daily or at each shift. Additionally, it is carried out whenever the ink is changed in a bench unit or a new batch is made up. A 100ml settlement flask, sometimes called a centrifuge tube, is used. The ink is agitated for 5 min and then a 100ml sample is poured into the settlement flask. The sample is allowed to settle for 60 min and the volume of solid is then read off the flask scale. Section 6.2 mentions the recommended concentration maxima and minima.

8.2

Fluorescent ink intensity Fluorescent inks should be discarded if there is evidence of fluorescence in the carrier fluid (supernatant liquid). This can happen due to over-vigorous agitation, causing the fluorescent dye to break off the magnetic particles. Also, a sample of unused agitated ink should be compared with a similar sample of in-use material under UV/A light. If there is any significant loss of brightness of the used material, it must be discarded.

9.3

Equipment performance check This test is carried out to find any changes that may have occurred during the day to day use of the equipment or materials. The test is carried out at the start of work or at shift change. British Standard 4069, Figures 2, 3 and 4 show detailed views of these test pieces, although the magnetic flow/coil often has five holes drilled transverse to the major axis at varying depths. To carry out the test the appropriate test piece is processed and the minimum ammeter readings which give satisfactory build-up of

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The Welding Institute particles at each hole, are established and noted. This is normally first done when everything else about the unit is in total control, e.g. ink renewed, ammeters calibrated, etc and is repeated for each current waveform. At each successive check the relevant hole should be visible at the appropriate amperage. There are variations on this method but the principle is followed in BS4069, Appendix B, paragraphs B2 and B4 and BS6072, Appendix C. 9.4

Viewing efficiency The output of the ultraviolet (UV-A) lamps used in magnetic particle inspection will deteriorate with age. In addition, the output can vary due to: a) b) c)

Displacement and tarnishing of the reflector Dirt and other contaminants on the filter Variations of the voltage to the lamp

It is therefore necessary to check the output of all UV-A lamps regularly. This check involves the use of a radiometer which will respond to radiation in the UV-A range (400nm-315nm) (nm = 1 nano metre = 10-9m). The test procedure is as follows. Position the radiometer with the detector at a distance of 400mm, or working distance, from the front surface of the lamp. If the reading at this distance exceeds the full scale of the meter, use such longer distances as will bring the reading to approximately 2/3 scale. Move the detector in a plane normal to the axis of the beam from the lamp until a maximum reading is obtained. Record on the lamp calibration label the radiometer reading, the distance of the lamp from the radiometer if greater than 400mm and the date. This test, repeated at regular intervals, will reveal any deterioration in performance or the need for maintenance of the lamp. Ultraviolet, UV-A, lamps should be changed if the output at working distance falls below 0.8mW/cm2 or 800µ W/cm2 (mW = milli watt, µW = micro watt) at test surface. The background light in an inspection area should be darker than 10 lux. If black ink is being used in white light conditions, the level of light at the work face should exceed 500 lux. This is equivalent to an 80W strip light at 1 metre.

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The Welding Institute 9.5

Magnetising units This is a general check for wear, abuse and general cleanliness.

9.6

Tank levels A surprising amount of ink, including solid particles, is carried off on components during testing. Ink level and strength checks are underrated items and are ignored at the inspector's peril.

9.7

Ultraviolet lamp maintenance A considerable loss of light output can be experienced because of dirty filters. Before condemning a lamp, clean it and the filter in a detergent solution.

9.8

Ammeters Must be checked and calibrated regularly with a meter traceable to national standards. Most major manufacturers will provide a service if ownership of a master meter is not considered economic.

9.9

Demagnetiser Often forgotten until something goes wrong. There does not seem to be a national standard but a major aerospace manufacturer recommends that demagnetisers should have a minimum internal field strength of 5.57kA/m. As with ammeters, the major MPI equipment manufacturers will provide a service when requested.

TWI Training & Examination Services – NDT 30M 51